In order to predict the deformation performance of composite compression struts with variable cross-section, the composite struts used in aerospace truss structure were taken as the research object in this paper. Firstly, wall panel of the composite struts was equivalent to an orthotropic plate, in approximation of an orthotropic strut, the principal direction was consistent with the axial direction of the strut, the composite strut could be approximated as an orthotropic strut. Based on the elastic deformation theory and small deformation assumption, the theoretical formula of equivalent axial compression stiffness was derived. Subsequently, accuracy of the theoretical formula of equivalent axial compression stiffness was verified by finite element method, results showed that: under conditions of different intermediate-radius and variable section-length, the deviation between values of the proposed theory and the finite element is within 3%; under the condition of different ply angles, the deviation between values of the theoretical and the finite element is within 2%.In conclusion, the theory can predict the axial deformation of composite struts with varying cross-section accurately.